Frame structure for allocating radio resources in a cellular wireless network

ABSTRACT

The capacity of a cellular wireless system is increased by operation of base stations or base station sectors arranged to re-use radio resource elements that are used by neighboring base stations or base station sectors, in conjunction with operation of relay stations, which are similarly arranged to re-use radio resource elements used by neighboring relay stations, and where the radio resource elements re-used by the relay stations are different to those used by the base stations. The relay stations provide coverage, particularly in the areas at the boundaries between the areas of coverage of base stations that suffer from interference between signals transmitted from the respective base stations. In addition, the relay stations generally increase the average available carrier to interference ratio compared with a system in which base stations alone are deployed. The scheme for the allocation of radio resource elements ensures in particular that interference is avoided between signals transmitted from a base station and signals transmitted from a relay station in radio resource elements allocated to control data.

PRIORITY CLAIM INFORMATION

The present application is a continuation of U.S. application Ser. No.14/606,577, filed Jan. 27, 2015, which is a continuation of U.S.application Ser. No. 13/717,952, filed Dec. 18, 2012 (now U.S. Pat. No.8,948,124), which is a continuation of U.S. application Ser. No.12/187,961, filed Aug. 7, 2008 (now U.S. Pat. No. 8,355,734); thedisclosures of each of the above-referenced applications areincorporated by reference herein in their entireties.

FIELD OF THE INVENTION

The present invention relates generally to wireless communicationsnetworks, and more specifically to a method and a system relating tomulti-hop or relay enhanced cellular wireless systems.

BACKGROUND OF THE INVENTION

Mobile telephony systems in which user equipment such as mobile handsetscommunicate via wireless links to a network of base stations connectedto a telecommunications network have undergone rapid development througha number of generations. The initial deployment of systems usinganalogue modulation has been superseded by second generation digitalsystems, which are themselves currently being superseded by thirdgeneration digital systems such as UMTS and CDMA. Third generationstandards provide for a greater throughput of data than is provided bysecond generation systems; this trend is continued with the proposal bythe Third Generation Partnership Project of the so-called Long TermEvolution system, often simply called LTE, which offers potentiallygreater capacity still, by the use of wider frequency bands, spectrallyefficient modulation techniques and potentially also the exploitation ofspatially diverse propagation paths to increase capacity (Multiple InMultiple Out).

Distinct from mobile telephony systems, wireless access systems havealso undergone development, initially aimed at providing the “last mile”(or thereabouts) connection between user equipment at a subscriber'spremises and the public switched telephone network (PSTN). Such userequipment is typically a terminal to which a telephone or computer isconnected, and with early systems there was no provision for mobility orroaming of the user equipment between base stations. However, the WiMaxstandard (IEEE 802.16) has provided a means for such terminals toconnect to the PSTN via high data rate wireless access systems.

Whilst WiMax and LTE have evolved via different routes, both can becharacterized as high capacity wireless data systems that serve asimilar purpose, typically using similar technology, and in additionboth are deployed in a cellular layout as cellular wireless systems.Typically such cellular wireless systems comprise user equipment such asmobile telephony handsets or wireless terminals, a number of basestations, each potentially communicating over what are termed accesslinks with many user equipments located in a coverage area known as acell, and a two way connection, known as backhaul, between each basestation and a telecommunications network such as the PSTN.

FIG. 1 shows a conventional wireless cellular network; in this example,the access links of base stations 2 a . . . 2 g are arranged in a socalled “n=3” frequency reuse pattern, that is to say that the availablewireless frequency spectrum is divided into three sub-bands f1, f2 andf3, in which n signifies the number of sub-bands. The area of coverageof each base station is divided into three sectors by the use ofdirectional antennas, and each of the sectors operates in a differentfrequency sub-band. In the example of FIG. 1, the sectors indicated byreference numerals 1 a, 1 b and 1 c associated with the base stationindicated by reference numeral 2 a operate in frequency sub-bands f1, f2and f3 respectively. It can be seen that the frequency re-use patternshown in FIG. 1 can be repeated without adjacent sectors operating atthe same frequency, thereby minimising interference between adjacentsectors. Sub-bands need not necessarily consist of contiguous blocks offrequencies, indeed there may be some advantage in interleaving thefrequencies in order to distribute the effect of frequency selectivefades. A frequency selective fade is a reduction in signal power due todestructive interference between multipath components. In cellularsystems employing orthogonal frequency division multiplexing (OFDM) suchas, for example WiMax or LTE, a sub-band will typically comprise anon-contiguous groups of sub-carriers; However, for clarity thesub-bands are often illustrated as being separate contiguous blocks, inwhich the numerical designation of a frequency has an arbitraryrelationship with the actual frequency at the physical layer.

FIG. 1 is a schematic diagram in which sectors 1 a, 1 b, 1 c are shownas hexagonal areas; in practice, geographical constraints andpropagation conditions will cause the area of coverage of each sector tobe irregular in shape and the areas of sectors to be unequal and thespacing of base stations will be determined by available sites and willnot necessarily correspond to the idealised situation shown, in FIG. 1.

There may be gaps in the area of coverage of a cellular system due toshadowing by the terrain or by interference between signals transmittedby base stations. Conventionally these gaps may be countered by the useof repeater stations that receive signals from a base station andre-transmit them into an area. where coverage is poor. However, arepeater station that simply retransmits all signals received within aband may cause interference that reduces coverage in other areas. Theinterference may be reduced by using a relay station instead of arepeater station; a relay station selects which signals to retransmit,typically transmitting to terminals within the area of poor coverage.

Typically, a relay station is a small low power base station with anomni-directional antenna, in contrast to a conventional base station,which typically operates with a higher transmit power than a relaystation, and typically employs directional antennas that are mounted ona tower to give an extensive area of coverage. The radio resource of thecellular wireless system may be used to relay backhaul traffic between arelay station and a conventional base station.

FIG. 2 shows a conventional relay station operating within a cellularwireless network; the operation may for example be in accordance withIEEE 802.16j. A user equipment 12 b is in communication with a relaystation 10. As the relay station 10 is not provided with a backhaul linkseparate from the cellular wireless resource, the relay station isallocated radio resource timeslots for use relaying backhaul data to andfrom the adjacent base station 2 a which is itself connected bymicrowave link to a microwave station 6 and thence to atelecommunications network 8 such as the public switched telephonenetwork. A user equipment 12 a is shown in direct communication with thebase station 2 a.

The relay station 10 may be deployed in an area partially obscured frombase stations by a geographical feature such as a hill or anotherobstruction such as buildings, or within a building to give coverage toparts of the building that experience a poor link or no coverage from abase station. The relay station 10 is positioned such that it cancommunicate with a base station, and also give coverage to an obscuredarea. Typically, the relay station is required to give coverage to asmaller area than that covered by a base station sector. Conventionally,relay stations are used to cover a small proportion of the area ofwireless coverage of the cellular wireless system, and the coverageareas of relay stations rarely overlap each other. In such aconventional low density deployment of relay stations, the allocation ofoperating frequencies to relay stations for communication with userequipment may be carried out in an ad hoc manner; it may be acceptableto re-use the frequency sub-band allocated to the base station sectorwithin which the relay station is deployed, if the area of overlap issmall between the coverage of the relay station and that of the basestation. Alternatively, a different sub-band may be allocated to therelay station from that allocated to the base station sector withinwhich the relay station is deployed. Provided that the area of coverageof the relay station is small, the potential for interference withsignals in other base station sectors and with signals from other relaystations may not be an issue.

However, there is potentially an advantageous use of relay stations forthe purpose of increasing the capacity of a wireless cellular network ingeneral, not limited to situations in which parts of the target areas ofcoverage are obscured from base stations. Such a general use of relaystations could potentially involve a high density deployment of relaystations within a base station sector, such that the coverage areas ofrelay stations may overlap with each other and also overlapsubstantially with the areas of coverage of base station sectors. Thepotential advantage of such a deployment is that relay stations wouldprovide local areas of signal reception in which the carrier tointerference ratio is improved over that provided by the base stationsalone.

However, it may be problematic to allocate frequency sub-bands to relaystations deployed within a conventional cellular wireless networkemploying n=3 frequency re-use in a way that does not result ininterference.

FIG. 3 illustrates the potential problems of deployment of relaystations 10 a . . . 10 c within a cellular wireless system using an n=3frequency re-use scheme, showing the area of coverage of two basestations 2 a, 2 b. Three relay stations 10 a . . . 10 c are deployedwithin the area of coverage of the base stations 2 a, 2 b, and threeuser equipments 12 a . . . 12 c are shown. A given user equipment 12 acan receive signals from both a base station 2 a and a relay station 10a. The user equipment 12 a will hand over to use whichever of the basestation 2 a and relay station 10 a provides the highest quality signal,which quality may be expressed in terms of carrier to interferenceratio. The aim of the hand over process is to increase the averagecarrier to interference ratio available within the area of coverage ofthe wireless cellular system and hence increase the traffic capacity,since the traffic capacity is related to the carrier to interferenceratio.

The allocation of frequency sub-bands to the system as illustrated byFIG. 3 is problematic, taking, for example, the case of the relaystation indicated by reference numeral 10 a. If this relay station 10 awere to be operated at frequency sub-band f1 as used by base stationsector 1 a, there is potential for interference between signalstransmitted from relay station 10 a and those transmitted by the basestation 2 a. If the relay station 10 a were to be operated at frequencysub-band f2, there is potential for interference between signalstransmitted from relay station 10 a and those transmitted by the basestation 2 b in the sector 1 e in communication with user equipment 12 b.In the case that the relay station lea were to be operated at frequencysub-band f3, there is potential for interference between signalstransmitted from relay station 10 a and those transmitted by the basestation 2 a in the sector 1 c in communication with user equipment 12 c.

FIG. 4 shows a conventional time frame structure allocating timeslotsalternately to access 14 a . . . 14 d and to backhaul, also referred toas “relay” 16 a . . . 16 c between a relay station 10 and an associatedbase station 2, in a system such as that illustrated in FIG. 2.

FIG. 5 shows an example of the conventional allocation of radio resourcewithin each of the access time slots 14 a . . . 14 d of the framestructure of FIG. 4. In a system not employing relays; the relaytimeslots may be absent, so that the access timeslots are contiguous intime. The radio resource is split in frequency into three sub-bands f1,f2, f3 for use in a n=3 re-use pattern such as that illustrated in FIG.1, it can be seen that each frequency sub-band is divided in the timedimension into control timeslots 18 a . . . 18 c and payload 20 a . . .20 c timeslots, and that the control timeslots 18 a . . . 18 c for thefrequency sub-bands f1, f2 and f3 coincide with one another in time.This coinciding in time occurs since a user equipment receiver 12 a, 12b is synchronised to the radio resource frame structure and the receiver12 a, 12 b is pre-programmed in accordance with the relevant cellularwireless standard such as, for example, the WiMax or LTE standard, toexpect to receive control data at the same time in each sub-band. Anexample of the data that would form part of the control timeslot 18 a .. . 18 c is the frame control header (FCH) in the 802.16 WiMax system.Similarly, in the case of LTE systems, there are control timeslots whichmay be located at various positions within the data frame; for example,control timeslots may be located at the beginning, middle and end of aframe. In general, control traffic may, for example, indicate the sizeof a frame and its start and stop addresses.

In order to receive the payload part 20 a . . . 20 c of a frame, it isnecessary to receive the respective control timeslots 18 a . . . 18 cassociated with the frame. It is thus particularly important that thecontrol timeslots 18 a . . . 18 c be protected from interference. In then=3 frequency reuse scheme illustrated by FIG. 1, interference betweencontrol timeslots of signals transmitted by adjacent sectors of anygiven base station 2 a is inherently minimised since, as has alreadybeen mentioned, adjacent sectors operate at different frequencysub-bands.

Typically, the information carried by the control timeslots will varybetween base stations and between base station sectors. Therefore,techniques that mitigate the effects of interference between basestations and between base station sectors by the intelligent combinationof potentially interfering signals that carry the same information arenot generally applicable for use with control timeslots. For example,soft handover and best server selection methods are generally notapplicable for use with the control timeslot as they would impose thelimitation that the information content of potentially interferingsignals would be the same.

While it may be possible to control the allocation of radio resourcewithin the payload part of the frame 20 a, 20 b, 20 c to avoidinterference between signals from the base station 2 and the relaystation 10, it is typically not possible to re-allocate the radioresource used for control data 18 a, 18 b, 18 c, since this is typicallydefined within the relevant cellular wireless standard to occur atpre-defined positions within the frame structure. User equipmentoperating to the relevant standard is thus pre-programmed to expectcontrol data at the pre-defined positions within the frame structure.Therefore, if the same sub-band is allocated to the relay station 10 asto the base station 2, there is the potential for interference to occurbetween control data transmitted from the base station 2 and controldata transmitted from the relay station 10.

In practice, relay stations are of most value when placed at theextremes of coverage of a base station sector, since it is here thataugmentation of coverage are most likely to be required, but it is alsoin this situation that interference is most likely to be caused. Inaddition, interference may be experienced between transmissions fromadjacent relay stations which may be operating in the same sub-band.

The use of a relay station within the area of coverage of a conventionalcellular wireless network using n=3 frequency re-use thus canpotentially cause interference with signals transmitted from basestations and with signals transmitted from neighbouring relay stations.

It is an object of the present invention to provide methods andapparatus which addresses these disadvantages.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, there isprovided a method of allocating radio resource within a frame structurein a cellular wireless network, the network comprising a plurality ofbase stations and a plurality of relay stations, the frame structurecomprising a plurality of first radio resource elements for thecommunication of control data, the method comprising allocating one ormore elements of the plurality of first radio resource elements forcommunication of control data by the base stations; and allocating oneor more different elements of the plurality of first radio resourceelements for communication of control data by the relay stations.

The benefit of this method is that interference is prevented betweencontrol data transmitted by a base station and control data transmittedby a relay station as received at user equipment. For example, a basestation may be equipped to give wireless coverage to a sector and arelay station may be operating within the area of coverage of the basestation. A user equipment may be pre-programmed to receive control datawithin a radio resource element or elements that may for example be acontrol timeslot within a frame structure allocating radio resourcewithin a network.

Preferably, the frame structure comprises a plurality of second radioresource elements for the communication of payload data, the methodcomprising allocating one or more elements of the plurality of secondradio resource elements for communication of payload data by the basestations; and allocating one or more different elements of the pluralityof second radio resource elements for communication of payload data bythe relay stations. This has the benefit that a high payload datacapacity can be transmitted by the re-use of radio resource elementsamongst base stations, and also by the re-use of different radioresource elements amongst relay stations. The proportion of payloadradio resource allocated to the base stations in relation to thatallocated to the relay stations may be in proportion to expected loadconditions, thus giving an efficient use of radio resource; theproportion need not be the same as that the proportion of control dataradio resource allocated to the base stations in relation to thatallocated to the relay stations, which is related to the control dataprotocol.

Preferably, the radio resource elements are distinguished by time and/orfrequency. For example, a base station may be equipped to give wirelesscoverage to a sector and a relay station may be operating within thearea of coverage of the base station. A user equipment may bepre-programmed to receive control data in a control timeslot within aframe structure allocating radio resource within a network. Theallocation of control data from the base stations and relay stations torespective frequency sub-bands that do not overlap prevents interferencebetween the control data from the base stations and that from the relaystations. Thus, control data from the base stations and relay stationsmay be allocated to respective timeslots or parts of timeslots that donot overlap in time thereby preventing interference between the controldata from the base stations and that from the relay stations. It will beapparent to one skilled in the art that a signal carried by a radioresource element will not interfere with signals carried by a differentradio resource elements, since radio resource elements are orthogonal toone another.

Advantageously, the wireless cellular network uses orthogonal frequencydivision multiplexing. For example, frequency sub-bands may comprisesets of orthogonal frequency division multiplexing (OFDM) subcarriers.As a result signals transmitted from a base station and a relay stationmay be received by means of a single OFDM receiver at a user equipmentthat receives a band encompassing the sub-bands used by the basestations and the relay stations. Thus, handover between the basestations and relay stations is facilitated. The allocation of OFDMsub-carriers to sub-bands can follow any pattern; subcarriers allocatedto different sub-bands may typically be interleaved in frequency. Thishas the benefit of distributing between sub-bands the effects of afrequency selective fade.

Preferably, the base stations transmit at a higher transmission powerthan the relay stations. The benefit is that the relay stations may becheaply constructed and used in a cost effective manner to improve theaverage carrier to interference ratio in areas where the carrier tointerference ratio provided by the base stations is limited.

Typically, the base stations employ directional antennas and the relaystations employ omni-directional antennas. As a result base stations maybe deployed to give sectorised coverage, that is to say that frequenciesare re-used between sectors of the azimuth plane surrounding a basestation. This implementation is efficient in minimising the number ofbase station sites, which is beneficial in that these may be relativelyexpensive high power devices with antennas mounted on towers. Bycontrast, relay stations may be small, cheap devices withomni-directional antennas which are preferably distributed in greaternumbers than base stations to improve the carrier to interference ratioin the area of wireless coverage, but which do not involve so greatinfrastructure costs per base station, due to their small, low powernature and typical lack of an antenna tower.

Advantageously, a base station employs dedicated backhaul, for example afibre or microwave link. A relay station typically uses radio resourceoccupying the same frequency band used for communication between therelay station and user equipment to provide backhaul of data to a basestation. The benefit is that relay stations can be deployed economicallyto provide augmentation of the coverage area of a base station or toimprove the carrier to noise ratio available within the coverage areawithout the expense and geographical limitation of providing dedicatedbackhaul. The relay stations may conveniently be operated substantiallyto the IEEE 802.16j standard.

Preferably, the network is configured such that regions of coverage inwhich signals from base stations suffer interference do not coincidewith regions of coverage in which signals from relay stations sufferinterference. The benefit is that the base stations may be operated witha n=1 frequency reuse scheme that gives high capacity close to basestations at the cost of interference at the boundaries between the areasof coverage of base station. Similarly, the relay stations can operate an=1 frequency reuse scheme at a separate frequency sub-band from thoseused by the base stations. Provided the boundary regions associated withthe base stations and the relay stations do not coincide, the networkcan provide efficient use of the spectrum as a user equipment canreceive signals from whichever base station or relay station isproviding the best carrier to interference ratio and hence the best datacapacity at a given location in the network.

The base stations and relay stations may be configured under the controlof a scheduler or network management system so that transmission andreception is arranged using radio resource elements allocated by thescheduler or network management system. For example, a sub-set of OFDMsub-carriers constituting a frequency sub-band to be used by basestations may be defined by a network management system and communicatedto the base station using a dedicated wired backhaul link. Theinformation relating to the sub-set of OFDM sub-carriers constituting afrequency sub-band to be used by relay stations may similarly becommunicated to relay stations from the network management system forexample via the base stations using timeslots dedicated for backhaul.

Radio resource allocation for payload may then be communicated to userequipment typically using a section of the pre-amble of a transmission,such as the frame control header or Map; this pre-amble constitutescontrol data, and uses radio resource whose relationship with a framestructure to which the handset is synchronised is pre-programmed intothe handsets. The radio resource used by the control data is typicallyarranged into duplicate sections, known as segments, defined in thestandard to which the wireless system operates, so that a user equipmentmay use any of the duplicate sections to receive control data. Theselection of which of the duplicate sections to use is made by thenetwork management system and communicated as described to the basestations and relay stations as appropriate; the user equipment simplylistens to the radio resource that it was pre-programmed to receive inaccordance with the wireless system standard.

Further features and advantages of the invention will become apparentfrom the following description of preferred embodiments of theinvention, given by way of example only, which is made with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a conventional wireless cellularnetwork;

FIG. 2 is a schematic diagram showing a conventional relay node incommunication with a base station;

FIG. 3 is a schematic diagram showing a potential method of operation ofrelay stations within a cellular wireless system;

FIG. 4 is a schematic diagram showing a conventional frame structureenabling timesharing between relay and access components;

FIG. 5 is a schematic diagram showing a conventional allocation of radioresource within the access portion of a frame structure;

FIG. 6 is a schematic diagram showing the operation of relay stationswithin a cellular wireless system according to an embodiment of theinvention;

FIG. 7 is a schematic diagram showing two examples of paths of a userequipment moving between the areas of wireless coverage of two basestations and two relay stations;

FIG. 8 is a schematic diagram showing two examples of paths of a userequipment moving between the areas of wireless coverage of two basestations;

FIG. 9 is a schematic diagram showing the carrier to interference ratioexperienced by a user equipment moving along the paths illustrated byFIG. 6 between the areas of wireless coverage of two base stations thatare transmitting in different frequency sub-bands;

FIG. 10 is a schematic diagram showing the carrier to interference ratioexperienced by a user equipment moving along the paths illustrated byFIG. 6 between the areas of wireless coverage of two base stations thatare transmitting in the same frequency sub-band;

FIG. 11 is a schematic diagram showing the carrier to interference ratioexperienced by a user equipment moving along the paths illustrated byFIG. 10 between the areas of wireless coverage of two base stations andtwo relay stations according to an embodiment of the invention;

FIG. 12 is a schematic diagram showing an allocation of radio resourceaccording to an embodiment of the invention within the access portion ofa frame structure:

FIG. 13 is a schematic diagram showing an allocation of radio resourceaccording to a further embodiment of the invention within the accessportion of a frame structure; and

FIG. 14 is a schematic diagram showing an allocation of radio resourceaccording to a yet further embodiment of the invention within the accessportion of a frame structure.

DETAILED DESCRIPTION OF THE INVENTION

In general, the present invention is directed to methods and apparatusthat are aimed to increase the capacity or coverage of a cellularwireless system by the use of relay stations. For clarity, the methodsand apparatus are described in the context of a high speed packet datasystem such as IEEE802.16 (WiMax) or LTE, but it will be appreciatedthat this is by way of example and that the methods and apparatusdescribed are not limited to these examples.

As a result of the problems of interference mentioned above, thepotential capacity increase offered by the use of relay stations withina cellular wireless network using a conventional n=3 frequency reusescheme is limited.

FIG. 6 illustrates a similar network to that illustrated in FIG. 5, withan improved frequency re-use scheme according to an embodiment of theinvention. In contrast to conventional cellular wireless systems inwhich the base stations operate within a frequency re-use scheme of n=3,in the system of FIG. 6 the base stations 2 a, 2 b operate within afrequency re-use scheme of n=1, that is to say, the base stations 2 a, 2b operate within the same frequency sub-band as each other. The relaystations 10 a . . . 10 b are allocated a different frequency sub-bandfrom that used by the base stations 2 a, 2 b. The relay stations thusalso operate with a n=1 frequency re-use pattern with respect to eachother. The scheme can be extended to a system comprising a plurality ofbase stations and a plurality of relay stations, the base stationsemploying a n=1 frequency re-use scheme with respect to each other andthe relay stations also using a n=1 frequency re-use scheme with respectto each other, in a different frequency sub-band to that used by thebase stations.

FIG. 7 illustrates an arrangement of two base stations, designated firstbase station 2 a and second base station 2 b with two relay stations 10a, 10 b placed between the first base stations according to anembodiment of the invention. A user equipment (not shown) is moved alonga first trajectory 22 that passes relatively close to the base stations2 a, 2 b and relay stations 10 a, 10 b indicated by a solid line andalso along a second trajectory 24 that passes relatively far from thebase stations 2 a, 2 b and relay stations 10 a, 10 b indicated by adashed line. The base stations 2 a, 2 b and relay stations 10 a, 10 bare shown arranged in line for illustrative purposes only; otherarrangements are possible.

FIG. 8 illustrates the arrangement of FIG. 7, but without the relaystations 10 a, 10 b, and thus—by way of a comparison—according toconventional arrangements. A user equipment (not shown) moves along afirst trajectory 22 that passes relatively close to the first and secondbase stations 2 a, 2 b indicated by a solid line and also along a secondtrajectory 24 that passes relatively far from the first and second basestations 2 a, 2 b indicated by a dashed line. FIG. 9 shows how thecarrier to interference ratio (C/I) received at the user equipmentvaries as the user equipment travels along the respective trajectories22, 24 of FIG. 8, in the case where the first and second base stations 2a and 2 b are operating at different frequencies designated arbitrarilyas f1 and f2 respectively, as would typically be the case within acellular wireless system operating with a n=3 frequency reuse scheme.The carrier to interference ratio experienced on the first trajectory 22is shown by the solid line 26 a, 26 b of FIG. 8 and that experienced onthe second trajectory 24 is shown by the dashed line 28 a, 28 b. Theinterference may originate from remote base stations re-using thefrequency in question.

The exact shapes of the C/I characteristics shown in FIG. 9 depend onthe propagation conditions; the curves shown are for illustrativepurposes. It should be noted that as the user equipment moves furtherfrom the first base station 2 a, the signal level received at frequencyf1 from the first base station 2 a typically falls and the interference,which is typically from distant base stations (not shown in FIG. 8)operating at the same frequency sub band as the first base station 2 a,will not fall on average, so that the carrier to interference ratiofalls as shown by curves 26 a and 26 b. As the user equipment terminalmoves closer to the second base station 2 b, the signal level receivedat f2 increases and the carrier to interference ratio similarlyincreases as shown by curves 26 a and 26 b. At the point where thecarrier to interference ratio is higher for the signals received fromthe second base station 2 b than for those received from the first basestation 2 a, the user equipment hands over from the first base station 2a to the second base station 2 b; the curve of carrier to interferenceratio against distance is thus the bold solid line 26 a, 26 b for thefirst trajectory 22 and the dashed line 28 a, 28 b for the secondtrajectory 24. It should be noted that typically, the handover pointoccurs above a 0 dB carrier to interference ratio; that is to say, thesignal power is greater than the interference power.

FIG. 10 shows how the carrier to interference ratio (C/I) received atthe user equipment varies as the user equipment travels along therespective trajectories 22, 24 of FIG. 8, in the case where the firstand second base stations 2 a, 2 b are operating in the same frequencysub-band designated arbitrarily as f1, as would typically be the casewithin a cellular wireless system operating with a n=1 frequency reusescheme. The carrier to interference ratio experienced on the firsttrajectory 22 is shown by the solid line 26 a, 26 b of FIG. 10 and thatexperienced on the second trajectory 24 is shown by the dashed line 28a, 28 b. It should be noted that, since both base stations operate inthe same sub-band, each base station appears as interference to theother. Therefore, at the point of handover, the signal and interferencewill be of at best equal power, that is to say the carrier tointerference ratio will be 0 dB at best. This will be true whatever thetrajectory between the base stations 2 a, 2 b; it can be seen from FIG.10 that the curves 26 a, 26 b relating to the first trajectory and thecurves 28 a, 28 b relating to the second trajectory both pass through a0 dB point.

The falling of carrier to noise ratio to 0 dB between base stationsoperating according to a n=1 frequency reuse scheme generally presents aproblem to the operation of a cellular base station since the trafficcapacity of a channel, to be shared between user equipments, is relatedto the carrier to interference ratio. While modern communication systemsmay be able to operate at a low capacity at 0 dB carrier to interferenceratio, this offers a great reduction in capacity compared with regionsof coverage away from transitions between base stations, and this isgenerally not acceptable. As a result, n=1 frequency re-use systems aregenerally not proposed for high capacity OFDM cellular wireless systemssuch as WiMax and LTE. However, a n=1 frequency reuse scheme offerspotentially a major advantage over a n=3 scheme, in that the wholefrequency band is available for use near a base station, rather thannominally a third of the band. As a result the capacity available nearto base stations and away from interfering base station can be veryhigh. It will, however, be appreciated that this potential benefit hasto be balanced against the problems described above that are encounteredat the interfaces between areas of coverage of base stations.

Embodiments of the invention provide a relief to the tension between therespective benefit and drawback by arranging relay stations to fill ingaps that would otherwise result in the coverage. This will now beexplained with reference to FIG. 11, which shows how the carrier tointerference ratio (C/I) received at the user equipment varies as theuser equipment travels along the respective trajectories 22, 24 of FIG.7, and thus according to an embodiment of the invention. In thisexample, and as described above, the first and second base stations 2 aand 2 b are operating at the same frequency sub-band designatedarbitrarily as f1, and the relay stations 10 a, 10 b are operating at adifferent frequency sub-band designated f3. The carrier to interferenceratio experienced on the first trajectory 22 is shown by the solid line26 a . . . 26 i of FIG. 11 and that experienced on the second trajectory24 is shown by the dashed line 28. It can be seen that the userequipment hands over to whichever base station 2 a, 2 b or relay station10 a, 10 b has the higher carrier to interference ratio. As a result, itcan be seen that carrier to noise ratio can be maintained above 0 dBprovided relay stations are positioned such that the 0 dB point 30 thatoccurs between base station 2 a, 2 b at f1 does not coincide with the 0dB point 32 that occurs between relay station 10 a, 10 b at f3.

FIG. 12 illustrates an allocation of radio resource within an accessportion 14 a of the data frame of FIG. 3, according to an embodiment ofthe invention.

Radio resource allocated to control data relating to communication froma base station sector to user equipments operating within the area ofcoverage of the base station sector is allocated a region S1 18 a, andradio resource allocated to control data relating to communication froma relay station to user equipments operating within the area of coverageof the relay station is allocated a region S3 18 c, within a differentfrequency sub-band within the same timeslot. It should be noted that thecontrol timeslot is broadcast to any user equipments within the area ofcoverage. Not all of the sub-bands available in a timeslot need be used,as can be seen by the presence of the hashed area 18 b. It may beconvenient to use frequency resource for the control data portioncorresponding to that defined in the relevant cellular wireless standardfor a n=3 re-use scheme; this has the benefit that operation isfacilitated with user equipment that is pre-programmed to expect controldata at these frequencies and timeslots within a data frame.

It is not necessary that the payload frequency resource 20 d, 20 e bedivided in frequency with the same split as is used for the control data18 a, 18 c. Indeed in order to make optimum use of payload capacity, thedivision in frequency resource between the portion of payload allocatedto the link from the base stations to the user equipment and the linkfrom the relay stations to the user equipment may be determined by anetwork management system in response to the relative demand forcapacity relating to data to be carried by the respective portions. Theproportion may be set on a fixed basis, or may be adaptive according tothe load conditions. The frequency resource as illustrated by FIG. 12may be applicable for example to a system built to a WiMax IEEE802.16standard.

FIG. 13 illustrates an alternative allocation of radio resource to anaccess portion of the allocation of radio resource, according to afurther embodiment of the invention.

Radio resource for control data relating to communication from a basestation to a user equipment is allocated regions indicated by referencenumerals 18 c, 18 e and 18 g, and radio resource for control datarelating to communication from a relay station to a user equipment isallocated to regions indicated by reference numerals 18 d, 18 f and 18h, within a different frequency sub-band within the same timeslots. Asfor the embodiment shown in FIG. 12, not all of the sub-bands availablein a timeslot need be used; indeed it may be convenient to use frequencyresource for the control data portion corresponding to that which aredefined in the relevant cellular wireless standard for a n=3 re-usescheme. Radio resource portions indicated by reference numerals 20 f and20 h are allocated to the data payload transmitted from base stations touser equipment, and radio resource portions indicated by referencenumerals 20 g and 20 i are allocated to the data payload transmittedfrom relay stations to user equipment.

FIG. 14 shows a variation of the frequency allocation illustrated byFIG. 13 according to a yet further embodiment of the invention in whichthe division of the payload frequency resource between portions 20 f, 20h relating to transmission from base stations to user equipment andportions 20 g, 20 i relating to transmission from relay stations to userequipment is determined in a similar manner to that of FIG. 12; to makeoptimum use of payload capacity, the division in frequency resourcebetween the portion of payload allocated to the link from the basestations to the user equipment and the link from the relay stations tothe user equipment may be determined by a network management system inresponse to the relative demand for capacity relating to data to becarried by the respective portions. The proportion may be set on a fixedbasis, or may be adaptive according to the load conditions.

The frequency resource as illustrated by FIG. 13 and FIG. 14 may beapplicable for example to a system built to a LTE standard.

It can thus be seen that embodiments of the invention provide a methodof increasing the capacity of a cellular wireless system by enablingbase stations to operate a n=1 frequency re-use scheme by the deploymentof relay stations to provide coverage at the boundaries between theareas of coverage of base stations that would otherwise suffer frominterference between signals transmitted from the base stations. Therelay stations are themselves deployed in a second n=1 frequency re-usearrangement operating in a different sub-band to that used by the basestations. It is found that the operation of the base stations and relaystations in combination according to this embodiment gives an efficientuse the radio resource in terms of increasing capacity compared to a n=3reuse scheme of base stations alone operating in the same band.

In arrangements according to embodiments of the invention, the areas ofpoor coverage by relay stations are arranged not to coincide with theareas of poor coverage by base stations. Signals used by relay stationsand signals used by base stations are arranged to be orthogonal so thatbase stations will only cause interference to other base stations andrelay stations will only cause interference to other relay stations;there will be no interference between base station signals and relaysignals. This will have the effect of de-correlating the locations ofthe holes in the base station coverage from the locations of the holesin the relay station coverage. Orthogonality between relay stationsignals and base station signals may be achieved by the use of separaterespective frequency bands.

Conventionally, some control signals may be required to be transmittedusing the same radio resources by all base stations and relay stations,because user terminals may expect to find them on specified channels. Insuch cases, mutual interference between base station and relay signalscan occur, to the detriment of the system as a whole. Specifically,additional holes in coverage may result in areas where both base stationand relay signals are present. According to a preferred arrangement, ina radio resource structure originally designed to support frequencyreuse of n=3, one sub-band each can be used for relay station controlsignals and base station control signals respectively. This maintainsorthogonality between base station and relay station signals, and basestations and relay stations each operate with a frequency reuse of n=1.This is preferable to operating base stations and relays with afrequency reuse of n=3 sharing the same channels, as then interferencebetween base station and relay signals can occur. Accordingly, relaystations may be operated in conjunction with base stations such that arugged control signal can be achieved for both relay stations and basestations while maintaining orthogonality so as to minimise mutualinterference and minimise the correlation of their respective coveragepatterns, so maximising capacity and coverage.

Embodiments of the invention are also applicable to multi-hop wirelesssystems, in which backhaul between a relay station and a base stationmay comprise backhaul via one or more further relay stations or basestations.

It is not required that relay stations use shared radio resource forbackhaul; relay stations could in principle use any method of backhaul.For example, a dedicated link to a telecommunications network could beprovided using conventional backhaul methods such as a fibre link or ahigh speed digital subscriber link.

It should be noted that an n=1 frequency re-use scheme is commonly usedby code division multiple access (CDMA) systems such as UMPTS release99. The effects of interference between base stations are mitigated insuch a CDMA system by soft handover, in which several base stations orbase station sectors simultaneously transmit the same payload data to auser equipment and the user equipment combines the payload data using acombining algorithm; this is a robust system but the data capacity iscompromised by the duplication of payload. High capacity OFDM cellularwireless systems such as WiMax and LTE are generally not designed toenable duplication of payload for such soft handoff and so n=1 frequencyre-use is conventionally not a feasible option.

Whilst embodiments of the invention have described handover in thecontext of the evolving LTE and WiMax systems, it will be appreciatedthat embodiments of the invention are also applicable to other cellularradio systems.

Furthermore, it will be apparent to those skilled in the art thatembodiments of the invention may be implemented by a computer readablemedium encoded with computer executable instructions for causing aprocessor to perform the method disclosed.

The above embodiments are to be understood as illustrative examples ofthe invention. It is to be understood that any feature described inrelation to any one embodiment may be used alone, or in combination withother features described, and may also be used in combination with oneor more features of any other of the embodiments, or any combination ofany other of the embodiments. Furthermore, equivalents and modificationsnot described above may also be employed without departing from thescope of the invention, which is defined in the accompanying claims.

The invention claimed is:
 1. A method for operating a user equipmentdevice, the method comprising: receiving, in a first time slotcomprising a first time interval for control and a second time intervalfor payload, first control information from a first base station withina first coverage area, wherein the first control information occurswithin the first time interval of the first time slot and a firstfrequency subset, wherein interference associated with second controlinformation from a second base station occurs in the first time intervalof the first time slot and the first frequency subset within a secondcoverage area, wherein the first and second coverage areas aresubstantially different; wherein third and fourth coverage areas,corresponding respectively to third and fourth base stations, overlapthe first coverage area in one or more parts of the first coverage area,wherein the third and fourth base stations do not interfere with thefirst base station in the one or more parts of the first coverage area;receiving first payload data in the second time interval of the firsttime slot from the first base station based on the first controlinformation; after handover from the first base station to the thirdbase station, receiving, in a second time slot comprising a first timeinterval for control and a second time interval for payload, fifthcontrol information from the third base station, wherein the fifthcontrol information occurs within a first time interval of the secondtime slot and the second frequency subset within the third coveragearea; and receiving second payload data in the second time interval ofthe second time slot from the third base station based on the fifthcontrol information; wherein signals are not received from a first setof base stations of a first type, the first set comprising the first andsecond base stations, within the second frequency subset; and whereinsignals are not received from a second set of base stations of a secondtype, the second set corresponding to low power base stations andcomprising the third and fourth base stations, within the firstfrequency subset.
 2. The method of claim 1, wherein the first basestation corresponds to a first cell, wherein the second base stationcorresponds to a second cell that neighbors the first cell, wherein thethird base station is positioned to cover a boundary region between thefirst cell and the second cell.
 3. The method of claim 1, wherein thefirst base station corresponds to a first cell, wherein the second basestation corresponds to a second cell that neighbors the first cell,wherein the third base station is positioned to cover a region of highinterference between the first cell and the second cell.
 4. The methodof claim 1, wherein the first frequency subset and the second frequencysubset are subsets of OFDM subcarriers.
 5. The method of claim 1,wherein the first payload data occurs within a third frequency subset,wherein the second payload data occurs within a fourth frequency subset,wherein the third frequency subset is not the same as the firstfrequency subset.
 6. A user equipment device comprising: an antennasubsystem; and a receiver coupled to the antenna subsystem, andconfigured to: receive, in a first time slot comprising a first timeinterval for control and a second time interval for payload, firstcontrol information from a first base station within a first coveragearea, wherein the first control information occurs within the first timeinterval of the first time slot and a first frequency subset, whereininterference associated with second control information from a secondbase station occurs is in the first time interval of the first time slotand the first frequency subset within a second coverage area, whereinthe first and second coverage areas are substantially different; whereinthird and fourth coverage areas, corresponding respectively to third andfourth base stations, overlap the first coverage area in one or moreparts of the first coverage area, wherein the third and fourth basestations do not interfere with the first base station in the one or moreparts of the first coverage area; receive first payload data in thesecond time interval of the first time slot from the first base stationbased on the first control information; after handover from the firstbase station to the third base station, receive, in a second time slotcomprising a first time interval for control and a second time intervalfor payload, fifth control information from the third base station,wherein the fifth control information occurs within a first timeinterval of the second time slot and the second frequency subset withinthe third coverage area; and receive second payload data in the secondtime interval of the second time slot from the third base station basedon the fifth control information; wherein signals are not received froma first set of base stations of a first type, the first set comprisingthe first and second base stations, within the second frequency subset;and wherein signals are not received from a second set of base stationsof a second type, the second set corresponding to low power basestations and comprising the third and fourth base stations, within thefirst frequency subset.
 7. The user equipment device of claim 6, whereinthe first base station corresponds to a first cell, wherein the secondbase station corresponds to a second cell that neighbors the first cell,wherein the third base station is positioned to cover a boundary regionbetween the first cell and the second cell.
 8. The user equipment deviceof claim 6, wherein the first base station corresponds to a first cell,wherein the second base station corresponds to a second cell thatneighbors the first cell, wherein the third base station is positionedto cover a region of high interference between the first cell and thesecond cell.
 9. The user equipment device of claim 6, wherein the firstfrequency subset and the second frequency subset are subsets of OFDMsubcarriers.
 10. The user equipment device of claim 6, wherein the firstpayload data occurs within a third frequency subset, wherein the secondpayload data occurs within a fourth frequency subset, wherein the thirdfrequency subset is not the same as the first frequency subset.
 11. Anon-transitory computer readable memory medium that stores programinstructions executable by a processor to: receive, in a first time slotcomprising a first time interval for control and a second time intervalfor payload, first control information from a first base station withina first coverage area, wherein the first control information occurswithin the first time interval of the first time slot and a firstfrequency subset, wherein interference associated with second controlinformation from a second base station occurs in the first time intervalof the first time slot and the first frequency subset within a secondcoverage area, wherein the first and second coverage areas aresubstantially different; wherein third and fourth coverage areas,corresponding respectively to third and fourth base stations, overlapthe first coverage area in one or more parts of the first coverage area,wherein the third and fourth base stations do not interfere with thefirst base station in the one or more parts of the first coverage area;receive first payload data in the second time interval of the first timeslot from the first base station based on the first control information;after handover from the first base station to the third base station,receive, in a second time slot comprising a first time interval forcontrol and a second time interval for payload, fifth controlinformation from the third base station, wherein the fifth controlinformation occurs within a first time interval of the second time slotand the second frequency subset within the third coverage area; andreceive second payload data in the second time interval of the secondtime slot from the third base station based on the fifth controlinformation; wherein signal are not received from a first set of basestations of a first type, the first set comprising the first and secondbase stations, within the second frequency subset; and wherein signalsare not received from a second set of base stations of a second type,the second set corresponding to low power base stations and comprisingthe third and fourth base stations, within the first frequency subset.12. The non-transitory computer readable memory medium of claim 11,wherein the first base station corresponds to a first cell, wherein thesecond base station corresponds to a second cell that neighbors thefirst cell, wherein the third base station is positioned to cover aboundary region between the first cell and the second cell.
 13. Thenon-transitory computer readable memory medium of claim 11, wherein thefirst base station corresponds to a first cell, wherein the second basestation corresponds to a second cell that neighbors the first cell,wherein the third base station is positioned to cover a region of highinterference between the first cell and the second cell.
 14. Thenon-transitory computer readable memory medium of claim 11, wherein thefirst frequency subset and the second frequency subset are subsets ofOFDM subcarriers.
 15. The non-transitory computer readable memory mediumof claim 11, wherein the first payload data occurs within a thirdfrequency subset, wherein the second payload data occurs within a fourthfrequency subset, wherein the third frequency subset is not the same asthe first frequency subset.